1. Field of the Invention
This invention relates to solid fuel burning power plants and, more particularly, it concerns a method and apparatus for converting solid fuel, such as coal, to a motive fluid for driving a gas turbine used to generate electric power.
2. Description of the Related Art
Recent developments in coal burning power plants include pressurized fluidized bed combustion/combined cycle systems by which gaseous products of combustion and steam are used to drive respective gas and steam turbines for the generation of electricity. In such systems, combustion efficiency is enhanced by operation of fluidized bed combustors under superatmospheric pressure while combustion temperatures are maintained within ranges at which sulphur is absorbed, formation of nitrogen oxides is limited, and the metallurgical limits of gas turbine operating temperatures are satisfied. The combination of energy conversion efficiency, environmental acceptability and abundance of high sulphur coal has generated a high degree of interest in such systems by the power plant industry.
It is well known in the fluidized bed combustor art that sulphur contained in coal can be removed by burning crushed coal and a sorbent such as limestone or dolomite in fluidized bed during combustion of the coal and removing the sulphur laden sorbent from the system as dry waste. The temperature at which sulphur is most effectively removed from coal in this manner is in the region of 1600.degree. F. Also, combustion at this temperature substantially avoid the formation of air polluting oxides of nitrogen.
In a first generation of pressurized fluidized bed/combined cycle systems, the fluidized bed is operated at superatmospheric pressure on the order of five to six atmospheres to enhance the combustion process without excessive air velocities. Also, the compressed hot flue gases from the combustion process are cleaned of particulate material and used directly as the motive fluid for driving the gas turbine of the combined cycle. The temperature of the combustion in the fluidized bed is maintained by heat exchange transfer from the bed to provide superheated steam for driving the steam turbine of the combined cycle system. A difficulty in encountered with such first generation systems is that power converted by the gas turbine is limited by the combustion temperatures and pressures of the system. Given the operating parameters of the system and the limits of steam turbine efficiency, any increased power plant capacity is attainable only by increasing already very large combustor and steam turbine components.
In a second generation of pressurized fluidized bed/combined cycle systems, the ratio of power developed by the gas turbine to that developed by the steam turbine is significantly increased by using a two-stage solid fuel superatmospheric combustion process in which coal is first burned substoichiometrically in a pressurized fluidized bed combustor called a "carbonizer" to convert a substantial percentage of carbon in the coal to combustible fuel gas. Sorbent is fed with the coal to the carbonizer for removal of sulphur. Char and calcium sulfide remaining from this conversion are then burned at above stoichiometric conditions in a circulating fluidized bed char combustor from which heat is extracted by circulating the fluidized bed contents of the char combustor through a fluidized bed heat exchanger for the development of superheated steam to drive the steam turbine and generate electricity.
Hot fuel and flue gases from the respective carbonizer and the char combustor are cleaned separately in hot gas particulate clean up systems and the cleaned gases are directed into a topping combustor where they are mixed with compressed air and burned at temperatures approaching but not exceeding the metallurgical limits of the gas turbine (2100.degree.-2300.degree. F.). Hot gases from the topping combustor at approximately the same temperature and under system superatmospheric pressure are then expanded in the gas turbine. Surplus of mechanical work generated by the gas turbine (i.e., remaining after compressor operation for pressurizing the system) is also converted into electricity. Gases exhausted from gas turbine are cooled down in a heat recovery portion of the steam cycle and then exhausted to the atmosphere.
The fluidized bed heat exchanger of the second generation system, by which combustion temperatures are controlled in the char combustor and in which superheated steam is generated, requires a low fluidizing velocity to avoid tube erosion and therefore a large fluidized bed cross-sectional area is required to accommodate heat transfer surface. As a result of these requirements and the need for operation at the superatmospheric pressure of the system, the fluidized bed heat exchanger is one of the largest components of the second generation system. In a 225 MW power plant, for example, this heat exchanger occupies a space of 32,000 cubic feet. Moreover, it represents approximately 12% of the total plant cost.
Also, the pressurized char combustor, which operates at about 210% excess air for a) maintaining the turbine inlet temperature of 2100.degree. F., and b) reliable burning of the char and calcium sulfide, is, in itself, a very large and tall component of the system (approximately 36,000 cubic feet for a 225 MW plant). The large size of the char combustor in the known second generation system is due to several factors, such as the need for a relatively high gas residence time at the high amount of excess air to attain above stoichiometric combustion of the char without excessive combustion gas velocities in the char combustors, the space required for the fluidized bed heat exchanger, and the amount of carbon containing char remaining from the carbonizer.
While the art relating to solid fuel burning combined cycle power plants has reached an advanced stage of development, there is need for improvement particularly in reducing component size and costs associated therewith.